1. Field of Invention
This invention relates to a polishing pad and a polishing apparatus using the polishing pad, and more particularly, to a CMP polishing pad for flattening and smoothing the surface of an insulating film or a metal film formed on a semiconductor wafer during a manufacturing process of ULSI circuits, and an apparatus using such a CMP polishing pad.
2. Description of the Related Art
As semiconductor circuits become highly integrated with more minute patterns, the number of steps required for the manufacturing process of such semiconductors has increased, each step becoming more complicated. The increased number of complicated steps often result in uneven surfaces of semiconductor devices manufactured by that process. Unevenness on the surface of a semiconductor device causes cutoff of the interconnections and local increase of the resistance, which further causes breakdown and decrease of the current capacity. The unevenness also deteriorates the durability of the insulating film, and causes undesirable leakage.
Along with the high integration of semiconductor circuits, the wavelength of a light source used in optical lithography becomes shorter, while the numerical aperture (NA) increases. This means that the focal depth of a semiconductor exposure apparatus decreases. In order to deal with the shallow focal depth, a semiconductor device must have a smoother and more uniform surface.
In other words, the current semiconductor manufacturing technique requires a surface smoothing technique, as shown in FIG. 7. FIG. 7 illustrates two examples of smoothing semiconductor surfaces.
In FIG. 7(a), an interlayer insulating film 42 made of SiO.sub.2 is formed on a silicon wafer 41. This interlayer insulating film 42 has an uneven surface immediately after the deposition, and the uneven surface is polished into a flat and smooth surface. In FIG. 7(b), a metal layer 43 made of Al is formed on the entire surface of a silicon wafer 41. The Al metal layer 43 is polished in order to form an embedded electrode pattern of combination of the presence and the absence of the metal layer 43.
In order to flatten the semiconductor surface, a chemical mechanical polishing method or a chemical mechanical planerization method (hereinafter, referred to as a CMP method) is preferably used.
The CMP method has been developed based on a mirror polishing method for silicon wafers. The CMP is performed typically using an apparatus illustrated in FIG. 6. This CMP apparatus comprises a grinder, which consists of a surface plate 31 and a polishing pad 32 attached to the surface plate 31, and a polishing head 33 for holding a wafer 34. The wafer 34 is in contact with the polishing pad 32 which is being rotated together with the surface plate 31. At this time, the wafer 34 is reciprocated in the radial direction of the surface plate 31, while the wafer 34 itself is also being rotated, whereby the wafer surface is polished. During this operation, a load is applied on the polishing head 33 from the above.
An abrasive 36 is supplied onto the polishing pad 32 from an abrasive supply nozzle 15. The abrasive 36 diffuses onto the polishing pad 35, and gets into the gap between the polishing pad 32 and the wafer 34 during the relative motion of these two elements, whereby the surface of the wafer 34 is polished. Thus, the mechanical motions of the polishing pad 32 and the wafer 34, and the chemical effect of the abrasive 36 mutually act on the surface of the wafer 34.
In the conventional polishing apparatus, a sheet-type polishing pad (or polisher) made of foamed polyurethane is typically used as the polishing pad 32.
However, sheet-type polishing pads made of foamed polyurethane have several problems. First, if a wafer is polished using such a sheet-type polishing pad, the edge of the wafer is apt to hang down. Second, sheet-type polishing pads are likely to deform due to the polishing pressure generated by a load on the polishing head. Third, if a sheet-type polishing pad is pasted onto the surface plate, unevenness of the adhesive prevents the polishing surface of the polishing pad from having a desired flatness. To be more precise, it is difficult for the polishing pad to achieve a profile irregularity less than .lambda.. Fourth, a sheet-type polishing pad is easily loaded with particles of polished wafer and, therefore, a dressing operation is required to file the polishing surface of the pad.
In order to solve these problems, a polishing pad made of epoxy resin as the major component has been proposed by, for example, Japanese patent application serial No. 8-115794.
As has been described in connection with FIG. 7, two different types of layers, an interlayer insulating film (e.g., an SiO.sub.2 layer) and a metal interconnection (e.g., an Al or W layer), are polished by a CMP process using different abrasives. The pH values of the abrasives greatly differ depending on which type of layer is polished. When polishing insulating films, an alkaline abrasive with a pH value of about 11 is used, while when polishing metal layers, an acid abrasive with a pH value of about 3 is used. Accordingly, the polishing pad must have both alkali-resistant and acid-resistant properties.
However, if aliphatic amine is used as a curing agent in a manufacturing process of a polishing pad made of epoxy resin as the major component, the polishing pad does not have an acid-resistant property. If fatty acid anhydride is used as a curing agent, the final product (i.e., the polishing pad) does not have an alkali-resistant property. Such a polishing pad is soluble and corrosive to the abrasive.
In addition, the hardnesses of the above-described two types of objects, namely, the SiO2 interlayer insulating film and the Al or W interconnection, are very different from each other. If the hardness of the polishing pad is smaller than that of the object to be polished, the wear of the polishing pad is greater than that of the object being polished. In this case, the correct shape and the precision of the polishing pad can not be maintained. If the polishing pad is harder than the object to be polished, then a small amount of impurity existing in the gap between the polishing pad and the object being polished easily causes scratches on the object surface. Therefore, the hardness of the polishing pad must be appropriately selected corresponding to the hardness of the object to be polished.
In response to this demand, the hardness of an epoxy resin polishing pad is adjusted by changing the mixture ratio of the epoxy resin and the curing agent, by changing the kind and the amount of the particulates dispersed in the epoxy resin and the curing agent, or by combination of these two methods.
However, if the mixture ratio of the epoxy resin and the curing agent is changed in order to adjust the hardness of the final product, the stoichiometric reaction ratio of the epoxy resin (which is the major component of the polishing pad) and the curing agent deviates from the optimum stoichiometric ratio and, as a result, the heat transition point lowers. In addition, the chemical resistance (i.e., the alkali resistance and the acid resistance) are also deteriorated by nonreacted resin.
If the hardness is adjusted by changing the kind and the amount of the particulates dispersed in the epoxy resin or the curing agent, the adjustable range is very limited. In addition, depending on the type and the amount of the particulates, uniform dispersion in the epoxy resin can not be achieved, and an undesirable distribution of hardness is caused in the polishing plane of the polishing pad.